1. Field of the Invention
The present invention generally relates to printing technology and in particular to techniques used for printing a polychrome graphic representation that may, exclusively or not, comprise texts and/or illustrations that are reproduced on support media of different shapes and types. The invention more particularly relates to means for detecting a possible color drift in polychrome reproductions with respect to an original polychrome graphic representation. One particularly advantageous application of the present invention is in processes for printing a polychrome graphic representation on support media in the form of canisters, and in particular metal canisters containing various types of products.
2. Background of the Invention
In the above application, there happens to occur a color drift during the printing of successive graphic representations on the canisters. This is due, among other things, to an alteration in the shades of ink. This alteration in shades is mainly caused by temperature variations in the ink ducts in the course of the printing process and is due to various factors such as the warming of the printing machine used.
This calls for a periodic comparison between the reproductions obtained and the original graphic representation, in view of modifying the relative proportions of the inks used so as to correct for any drift in the observed shades. Up until now, this type of inspection was carried out by personnel employed to visually detect color drifts.
Not only do such inspections prove to be difficult and tiring, and call for considerable experience and visual acuity, but it also appears that color drifts are all the more difficult to detect when the graphic representation is in the form of a figure, painting or photograph showing an essentially figurative element, such as a face or landscape. Indeed, in such cases people performing the inspection tend to consider the general impression given by the reproduction and cease to observe the different shades in the details forming the figurative elements. As a consequence color drifts are only detected when they become obvious. A result of this late detection is the rejection of canisters bearing such reproductions and having shades that do not conform with those of the original representation.
It is therefore one of the aims of the present invention to overcome this drawback by providing a process for visually detecting a color drift as soon as it occurs during the course of printing an original polychrome graphic reprsentation. Another object of the invention is to provide a process that dispenses with the figurative element depicted on both the reproduction under inspection and the original representation, to show up any difference in color that may occur during the course of printing.
A further object of the present invention is to provide a process that facilitates the comparison of colors between an original graphic representation and a reproduction under inspection.